Re: [RFC, PATCH 09/12] KVM: TDX: Preallocate PAMT pages to be used in page fault path

[Chao Gao <chao.gao@intel.com>]

On Fri, May 02, 2025 at 04:08:25PM +0300, Kirill A. Shutemov wrote:
>Preallocate a page to be used in the link_external_spt() and
>set_external_spte() paths.
>
>In the worst-case scenario, handling a page fault might require a
>tdx_nr_pamt_pages() pages for each page table level.
>
>Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>---
> arch/x86/include/asm/kvm_host.h |  2 ++
> arch/x86/kvm/mmu/mmu.c          | 10 ++++++++++
> 2 files changed, 12 insertions(+)
>
>diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>index 91958c55f918..a5661499a176 100644
>--- a/arch/x86/include/asm/kvm_host.h
>+++ b/arch/x86/include/asm/kvm_host.h
>@@ -849,6 +849,8 @@ struct kvm_vcpu_arch {
> 	 */
> 	struct kvm_mmu_memory_cache mmu_external_spt_cache;
> 
>+	struct kvm_mmu_memory_cache pamt_page_cache;
>+
> 	/*
> 	 * QEMU userspace and the guest each have their own FPU state.
> 	 * In vcpu_run, we switch between the user and guest FPU contexts.
>diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>index a284dce227a0..7bfa0dc50440 100644
>--- a/arch/x86/kvm/mmu/mmu.c
>+++ b/arch/x86/kvm/mmu/mmu.c
>@@ -616,6 +616,15 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> 		if (r)
> 			return r;
> 	}
>+
>+	if (vcpu->kvm->arch.vm_type == KVM_X86_TDX_VM) {

The check for vcpu->kvm->arch.vm_type == KVM_X86_TDX_VM is identical to
kvm_has_mirrored_tdp() a few lines above.

>+		int nr = tdx_nr_pamt_pages(tdx_get_sysinfo());

Since you're already accessing tdx_sysinfo, you can check if dynamic PAMT is
enabled and allocate the pamt page cache accordingly.

>+		r = kvm_mmu_topup_memory_cache(&vcpu->arch.pamt_page_cache,
>+					       nr * PT64_ROOT_MAX_LEVEL);
>+		if (r)
>+			return r;
>+	}
>+
> 	return kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_page_header_cache,
> 					  PT64_ROOT_MAX_LEVEL);
> }
>@@ -626,6 +635,7 @@ static void mmu_free_memory_caches(struct kvm_vcpu *vcpu)
> 	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_shadow_page_cache);
> 	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_shadowed_info_cache);
> 	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_external_spt_cache);
>+	kvm_mmu_free_memory_cache(&vcpu->arch.pamt_page_cache);
> 	kvm_mmu_free_memory_cache(&vcpu->arch.mmu_page_header_cache);
> }
> 
>-- 
>2.47.2
>